There have been conventionally proposed a number of amorphous semiconductor films such as amorphous silicon film which is, for example, composed of amorphous silicon compensated by hydrogen (H) and/or halogen (X) (fluorine, chlorine, etc.) [which will be hereinafter referred to as "A-Si(H,X)"] for use as element members for a semiconductor device, electrophotographic photosensitive member, image input line sensor, image pickup device, photovoltaic device, other various electronic elements and optical elements. Some of these amorphous semiconductor films have been practically used.
It is known that such deposited films may be formed by a plasma CVD method, that is by the glow discharge in a reaction gas using direct current, high-frequency or microwave to thereby form a thin deposited film on a substrate of glass, quartz, heat resistant synthetic resin, stainless steel, aluminum, etc. Along with this, various apparatuses for carrying out such process have been proposed.
In particular, a plasma CVD method using microwave glow discharge decomposition, namely, a microwave plasma CVD method (which will be hereinafter referred to as "MW-PCVD method") has been recently noticed on the industrial scale.
In the case that a reaction gas is decomposed using a high-frequency energy to form a deposited film, for instance, for an electrophotographic photosensitive member, the formation of such deposited film by way of the conventional plasma CVD method is carried out as follows:
There is shown a representative apparatus for forming the deposited film in FIG. 6. Referring to FIG. 6, the apparatus includes a high-frequency power source 601, a matching box 602, a diffusion pump and/or mechanical booster pump 603, a motor 604 for rotating an Al substrate 605, a heater 606 for heating the Al substrate 605, a gas feed pipe 607, a high-frequency induction cathode 608, a shield member 609, an electric power source 610 for the heater, valves 621-625 and 641-645, mass flow controllers 631-635, regulators 651-655, reservoir 661 for H.sub.2 gas reservoir 663 for silane (SiH.sub.4), reservoir 664 for NO, and reservoir 665 for methane (CH.sub.4).
A process for the formation of the deposited film by using the apparatus shown in FIG. 6 is carried out in the following manner. First, main cocks of all the reservoirs 661-665 are closed, and all of the mass flow controllers and the valves are opened. Then, the diffusion pump 603 is operated to evacuate the inside of the deposition chamber to 10.sup.-7 Torr. At the same time, the heater 601 is actuated to heat the Al substrate 605 to about 250.degree. C., and it is maintained at this temperature. Then, the valves 621-625, 641-645 and 651-655 are all closed, and the main cocks of the cylinders 661-665 are all opened. Then, the diffusion pump 603 is replaced by a mechanical booster pump. A secondary pressure of the valves 651-655 with regulators is set to 1.5 kg/cm.sup.2. The mass flow controller 631 is set at a predetermined value. Then, the valve 641 is opened, and subsequently the valve 621 is opened to introduce the H.sub.2 gas into the deposition chamber. Then, in the same manner, the reaction gas as required, e.g., the SiH.sub.4 gas is introduced into the deposition chamber.
When the inner pressure of the deposition chamber becomes stable, the high-frequency power source 601 is switched on to generate a high frequency of 13.56 MHz, thereby generating glow discharge between the Al substrate 605 and the cathode 608. Thus, the deposited film is formed on the Al substrate 605.
In the formation of the deposited film by using the apparatus shown in FIG. 6, an effective power of the high frequency in the deposition chamber is controlled by reading an incident power and a reflected power indicated by a wattmeter (not shown) attached to the high-frequency power source 601 and obtaining a difference between the incident power and the reflected power.
In FIG. 7(A), there is shown a representative MW-PCVD apparatus for forming a deposited film, for instance, for electrophotographic photosensitive member.
FIGS. 7(B) and 7(C) are a vertical sectional view and a horizontal sectional view of the apparatus shown in FIG. 7(A), respectively.
Referring to FIGS. 7(A) to 7(C), the apparatus includes a reaction chamber 701, a microwave transmissive window 702 made of alumina ceramics or quartz, a waveguide 703, an exhaust pipe 704, a heater 707' installed in a substrate holder 707 for heating a substrate 705 and maintaining the substrate at a predetermined temperature, and a reaction gas supply tube 708. The reaction gas supply tube 708 is provided with a plurality of gas liberation nozzle 708' being directed to discharge space 706.
The reaction chamber 701 is so designed as to start discharge by self-exciting discharge without using a discharge trigger or the like, and it has a cavity resonator structure capable of resonating with an oscillation frequency of a microwave power source.
The formation of a deposited film by using the apparatus shown in FIGS. 7(A) to 7(C) is carried out as follows:
The inside of the reaction chamber 701 is evacuated through the exhaust pipe 704, and the heater 707' is actuated to heat the substrate 705 to a predetermined temperature and it is maintained at this temperature. Then, the substrate 705 is rotated by a drive motor (not shown) at a desired constant speed. Then, in the case of forming an amorphous silicon deposited film, raw material gases such as silane gas and hydrogen gas are fed through the gas supply tube 708, and the gas liberation nozzles 708' into the reaction chamber 701. At the same time, a microwave having a frequency of 500 MHz or more, preferably, 2.45 GHz is caused from the microwave power source 711. The microwave is supplied through the waveguide 703 and the microwave transmissive window 702 into the reaction chamber 701. Thus, the raw material gases thus introduced into the reaction chamber 701 is excited and decomposed with the action the microwave energy to cause neutral radicals, ions, electrons, etc. Then, these are reacted with each other to form a deposited film on the substrate 705.
In the process for the formation of the deposited film by decomposing the raw material gases as mentioned above, an effective power of the microwave to be introduced into the reaction chamber 701 is controlled by reading an incident power and a reflected power indicated by a wattmeter (designated by reference numeral 710 in FIG. 7(B)) provided on the way of the waveguide 703 and obtaining a difference between the incident power and the reflected power as an actual power. Then, the actual power is adjusted by a stab tuner 709 so as to minimize the reflected power. However, as the adjustment of the actual power is manually carried out, it is difficult to do it as expected.
Further, as the microwave has a high oscillation frequency, that is, it has a short wavelength in contrast to the high-frequency wave as mentioned previously, the microwave is reflected on the microwave transmissive window 702, the substrate 705, the gas supply pipe 708, etc. Thus, many reflective surfaces are present to cause multiple reflection. Accordingly, the accurate control of the effective power of the microwave according to the difference between the incident power and the reflected power is rendered more difficult than the case of formation of the deposited film by the high frequency.
Furthermore, as the discharge proceeds, there will occur a difference in quantity of permeation of the microwave due to heating of the microwave transmissive window 702, A-Si(H,X) film will deposit also on the window 702, the gas supply pipe 708. As a result, the resonance condition between the microwave permeated through the microwave transmissive window 702 and the reaction chamber 701 is slipped to cause ununiform discharge in the reaction chamber 701 and also to cause a reduction in the actual effective power to be introduced into the reaction chamber 701 through an apparent effective power by the difference between the incident power and the reflected power indicated by the wattmeter 710 is identical. And, should the formation of the film be carried out under the above condition, it is difficult to obtain a desired deposited film usable for an electrophotographic photosensitive member, since the resulting deposited film unavoidably becomes ununiform in thickness and film quality.
As a result, the resulting electrophotographic photosensitive member cannot provide satisfactory functions as required. In the case of mass production, a yield is rendered greatly low.
Thus, in the formation of the deposited film by way of the MW-PCVD method, the control of the effective power of the microwave to be introduced into the reaction chamber upon the difference between the incident power and the reflected power is more problematic than in the case of the formation of the deposited film by high frequency. In view of the above, there is an increased demand to provide a proper method for controlling the effective power of the microwave.